Data converter with background auto-zeroing via active interpolation

ABSTRACT

An analog-to-digital data converter converts an input signal to corresponding digital signals. The data converter includes two sets of comparison units arranged in an interlaced sense to alternatively analyze the input signal, generate digital signals corresponding to the result of comparing the input signal and reference signals. Each of the comparison units has a positive output and a negative output, and the digital signal is generated by the negative output and the positive output of the comparison units in a differential way. When the comparison units of one set are performing auto-zeroing, the comparison units of the other set perform the data conversion to generate corresponding digital signals.

BACKGROUND OF INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a data converter, and more particularly, an analog to digital data converter using active interpolation to realize background auto-zeroing.

[0003] 2. Description of the Prior Art

[0004] Using digital signals for information transmitting, processing and storing is an important foundation of the information industry. A digital signal is basically composed of a high state and a low state, so the digital signal has a high noise tolerance. In addition, the digital signals can be processed in a modular design. Therefore, circuits for transmitting, processing, and storing digital signals are key issues in the information industry.

[0005] All signals are essentially analog signals that vary continuously, such as human voices, natural light, and so on. A data converter, which can convert analog data to digital, is needed for processing analog signals into digital form. Digital signals are really continuous analog signals. In a digital signal, while the waveform of the signal varies, for instance from a high state to a low state, a transient state exists. The transient state comprises a rising edge and a falling edge. Thus, the waveform of the digital signal is not a perfect square wave. Digital signals are less ideal when the digital signals are frequently switched between different states. For processing the non-ideal digital signals, an analog to digital data converter is needed to obtain digital signals with more ideal characteristics. For this purpose, data converters must operate rapidly and continuously to obtain high frequency digital signals in real time.

[0006] Please refer to FIG. 1 of a schematic diagram of a prior flash data converter 10, which is used to convert an analog input signal Vin to a corresponding digital signal. The data converter 10 comprises a voltage dividing circuit 12, an encoding circuit 16, and a plurality of comparison units 14. In FIG. 1, eight comparison units are shown for instance. The voltage dividing circuit 12 comprises a plurality of resistors such as Ra, Rb, and Rc for dividing a voltage Vdc into different reference voltages Vr1 to Vr8 respectively at each node. Each of the comparison units 14 comprises an amplifier 18 and a latch circuit 19. The amplifier 18 receives the reference voltage generated by the voltage dividing circuit 12, and an input signal Vin for amplifying the difference between these two input signals to generate a corresponding signal to the latch circuit 19. The latch circuit 19 is triggered by a clock vclock to convert the output signal of the amplifier 18 to a digital signal in a high or low state. This converted digital signal is output to the encoding circuit 16. The encoding circuit 16 processes (ex. Correct) and encodes the digital signals generated by the comparison units 14.

[0007] Please refer to FIG. 2 of a timing diagram of the clock vclock, the input signals Vin, and the output digital signals of each comparison unit while the prior data converter 10 operates. The transverse axis in FIG. 2 is time. When the analog input signal Vin reaches the data converter 10, the amplifier 18 compares the input signal Vin with the corresponding reference voltage and outputs a comparison result to the latch circuit 19. According to the comparison result and a trigger of the clock vclock, the latch circuit 19 outputs a digital signal in a high state, which is shown by 1, or a low state, which is shown by 0. For example, at time t1, if the input signal Vin is less than the reference voltages Vr1 and Vr2 but more than the reference voltages Vr3 to Vr8, then the latch circuit 19 is triggered by the negative edge of the clock vclock (which is shown by arrows) and outputs digital signals as 0, 0, 1, 1, 1, 1, 1, and 1. In this manner, the input signal Vin at time t1 can be converted to a digital signal (0, 0, 1, 1, 1, 1, 1, 1). The encoding circuit 16 can encode the digital signal in advance, such as 011.

[0008] For the purpose of converting input analog signals to digital signals correctly, each amplifier must respond to the relationship between the input signal Vin and the reference voltage correctly. In a real circuit, each comparison unit generates an offset voltage due to the non-ideal characteristics of the device. This means the comparison unit adds the offset voltage to the input signal Vin and then compares the modified input signal Vin with the reference voltage. Thus, the comparison is not performed in an ideal operation situation. Additionally, if each comparison unit has different offset voltages, the data converter 10 is affected, and converts signals incorrectly.

[0009] To adjust the offset voltage in the comparison units, an auto-zeroing process is used to solve the problem. Please refer to FIG. 3 of a schematic diagram of a data converter 20 in the prior art. The data converter comprises a voltage dividing circuit 22 for providing reference voltages Vr1 to Vr4, four comparison units 24A to 24D, auxiliary circuits 26A and 26B, and an encoding circuit 28. The comparison units 24A to 24D have the same structure. The comparison unit 24A comprises four switches SP1, SP2, SP3 and SP4, a differential amplifier Ka with one output end and two input ends, a capacitor C0, and a latch circuit Ja. The switches SP1 to SP4 are controlled by a control signal vc1 and an inverted signal of the control signal vc1, shown as {overscore (vc1)}. The amplifier Ka comprises a feedback circuit controlled by the switch SP3 electrically connected to the input end P11 and the output end P12. The other input end of the amplifier Ka is electrically connected to a common mode voltage V0. The latch circuit Ja is triggered by the clock vclock. Each comparison unit is electrically connected to a resistor Rc.

[0010] The data converter 20 operates as following. The amplifier compares the input signal Vin with the reference voltage and outputs the comparison result to the latch circuit. The latch circuit is then triggered by the clock vclock to generate a digital signal. To compensate the offset voltage generated in the amplifiers, the switches in each comparison unit conduct or open to allow each comparison unit to perform a comparing process or an auto-zeroing process alternatively. The comparison unit 24C shown in FIG. 3 is in the auto-zeroing process. Please notice that the switches in the comparison unit 24C can conduct the feedback circuit of the amplifier Kc, and the capacitor can be also electrically connected to a corresponding reference voltage Vr3. At this time, the reference voltage Vr3 charges the capacitor C0 via the switch SP2 in the comparison unit 24C. When the feedback circuit conducts, a close loop is formed at the node P31 so that the amplifier Kc is virtually grounded to the common mode voltage V0. Thus, the charge amount in the capacitor C0 is dependent on the reference voltage Vr3. The charge amount is enough to compensate the offset voltage of the amplifier Kc and the purpose of auto-zeroing is achieved.

[0011] After the auto-zeroing process, each comparison unit switches to a comparing process and compares the input signal with the reference voltage. Then, a corresponding digital signal is output from the latch circuit. For example, the comparison unit 24A shown in FIG. 3 is performing a comparison process. Please notice that the switch SP3 is opened so that the feedback circuit does not conduct and make the amplifier Ka in an open loop. The signal Vin is input to the capacitor C0 via the switch SP1 so that voltages on both terminals of the capacitor C0 are modified. Since the charge amount of the capacitor C0 in the auto-zeroing process corresponds to the reference voltage, the voltage on node P11 corresponds to a comparison result between the input signal Vin and the reference voltage. This comparison result is sent to node P12 by the amplifier Ka. Finally, the comparison result is output to the latch circuit Ja via the switch SP4 and converted to a digital signal according to the trigger of the clock vclock. Thus, the purpose of comparing the input signal Vin and the reference voltage, and converting the input signal Vin, is achieved.

[0012] Next, please refer to FIG. 4 of a timing diagram of the control signals vc1 to vc4 and the clock vclock in the comparison units of the data converter 20. The transverse axis in FIG. 4 is time. Among the control signals vc1 to vc4, the signals in the high state control corresponding switches to conduct, and the signals in the low state control corresponding switches to open. The negative edge of the clock vclock triggers the latch circuit to latch digital signals. At time T2, the control signal vc3, which is in the high state, controls all switches in the comparison unit 24C to keep the comparison unit 24C in an auto-zeroing process during period Tz. At this time, the control signals vc1, vc2 and vc4, which are in the low state, keep the comparison units 24A, 24B, and 24D in the comparing processes respectively. Thus, the input signal Vin is compared with the reference voltage in each comparison unit and converted to a digital signal.

[0013] At time t2, the data converter 20 has only three comparison units in performing the comparing process. The comparison unit 24C performs the auto-zeroing process and therefore no comparison result is available. However, the comparison unit 24B is electrically connected to the comparison unit 24D via the resistor Rc. A comparison result for the comparison unit 24C in the auto-zeroing process is generated in an interpolating method. The theory of the interpolating method is illustrated in FIG. 5 of a schematic diagram of converting curves of amplifiers K1 in the comparison units 24B to 24D. The transverse axis in FIG. 5 is the input voltage of the amplifier K1 that is differentially input. The longitudinal axis is output voltage of the amplifier K1. The converting curves TP2 to TP4 correspond to the amplifier in the comparison units 24B to 24D respectively. The voltage Vk is a standard voltage for a latch circuit K2 to output a high or low digital signal. That means if the output voltage of the amplifier is higher or lower than the standard voltage Vk, the corresponded latch circuit generates a digital signal in a high or low state.

[0014] As shown in FIG. 3, when the comparison unit 24C is in the auto-zeroing process, the input of the latch circuit jc, which is the voltage at the node P33, is dependent on the outputs of the amplifier Kb and Kd. When the comparison unit 24C performs the auto-zeroing process, the voltage of the comparison unit 24C is the interpolated value of the output voltages of the amplifiers Kb and Kd. Thus, the voltage characteristic at node P33 is an average of the converting curves TP2 and TP4 along the longitudinal axis in FIG. 5. Using the average of the converting curves TP2 and TP4 to obtain another converting curve TP3 z is shown by an arrow 27. If the converting curves TP2 and TP4 of the amplifiers Kb and Kd are both approximately linear and the reference voltage Vr3 is designated as an average value of the reference voltages Vr2 and Vr4, the average converting curve TP3 z intersects the output voltage Vk on the longitudinal axis at the reference voltage Vr3 on the transverse axis.

[0015] As described above, when the comparison unit 24C performs the auto-zeroing process, the converting curve TP3 of the amplifier Kc is replaced by the converting curve TP3 z that is obtained from the amplifiers Kb and Kd. The converting curve TP3 z is not equal to the converting curve TP3. However, if the converting curve TP3 z and the voltage Vk on the longitudinal axis intersect at the reference voltage Vr3 on the transverse axis, the converting curve TP3 z at the node P33 can drive the latch circuit Jc. That means if the voltage at the node P33 is higher or lower than the standard voltage Vk, the corresponding latch circuit generates a digital signal in a high or low state respectively. Although the comparison unit 24C in the auto-zeroing process does not compare the input signal Vin with the reference voltage at the same time, a comparing result can be obtained by the OLE_LINK1 interpolation OLE_LINK1 of the adjacent comparison units.

[0016] As shown in FIG. 4, the data converter 20 has only one comparison unit in the auto-zeroing process at any time. The two adjacent comparison units, via the resistor network that is composed of the resistor Rc, interpolate the comparison result of the comparison unit in the auto-zeroing process. If the comparison unit 24A or 24D perform the auto-zeroing process, the comparison result is interpolated by the auxiliary circuits 26A or 26B and the comparison unit 24B or 24C.

[0017] While the data converter 20 converts analog signals to digital signals, each comparison unit performs the auto-zeroing process in turn. While one comparison unit is in the auto-zeroing process, its comparison result comes from other comparison units by an interpolating method via the resistor network, so that the data converter 20 operates with full functionality. However, a transient effect happens while the resistor network interacts with the capacitors in the data converter 20. When the comparison units generate a replacement comparison result via the resistor network, signals are delayed due to the RC constant so that the the input bandwidth of conventional data converter 20 is limited. The resistor network of the resistor Rc also increases the complexity and costs of circuit design and manufacture.

[0018] Additionally, the converting curve of each amplifier must have a large linear range so that the converting curve can be interpolated correctly. As shown in FIG. 5, the converting curve TP2 and TP4 near the reference voltage Vr3 has better be linear so that the converting curve TP3 z, which intersects with the voltage Vk at the reference voltage Vr3, can be averaged correctly. If the converting curves TP2 and TP4 near the reference voltage Vr3 are not linear, the curve TP3 z replaces the converting curve TP3 incorrectly. As known, the above paragraph is suitable for some special cases, for example, 2× interpolation. In other cases, the amplifier needs not to be linear near Vr3. The accuracy of the interpolated Vr3 depends on the odd symmetry of the I/O characteristics of the amplifier. The comparison units also perform the auto-zeroing process one by one, and thus different control signals for each comparison unit are required. For instance, four control signals are required for the four comparison units shown in FIG. 4. The circuit design is complicated.

SUMMARY OF INVENTION

[0019] It is therefore a primary objective of the claimed invention to provide a data converter with background auto-zeroing via an active interpolation to solve the above-mentioned problem.

[0020] According to the claimed invention, the data converter comprises two sets of comparison units arranged in an interlaced sense to alternatively accept the input signal to generate digital signals corresponding to the result of comparing the input and reference signals. The digital signal is generated by the comparison units in a differential way. When the comparison units of one set are performing auto-zeroing, the comparison units of the other set perform the data conversion to generate corresponding digital signals. The digital signals of the comparison unit of the set in auto-zeroing are generated in active interpolation based on the digital signals from the comparison units of the other set in data converting.

[0021] It is an advantage of the claimed invention that the comparison units of one set are performing auto-zeroing, and the comparison units of the other set perform the data conversion to generate corresponding digital signals simultaneously. In addition, the active interpolation in the claimed invention simplifies the circuit layouts so that the design and manufacturing time and costs are reduced. Moreover, result achieved by the active interpolation in the claimed invention is less sensitive to the non-linear characteristic of the converting curves.

[0022] These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0023]FIG. 1 is a schematic diagram of a prior art data converter.

[0024]FIG. 2 is a timing diagram of related signals of FIG. 1.

[0025]FIG. 3 is a schematic diagram of another prior art data converter.

[0026]FIG. 4 is a timing diagram of related signals and clock of FIG. 3.

[0027]FIG. 5 is a schematic diagram of related converting curves of FIG. 3.

[0028]FIG. 6 is a schematic diagram of a data converter according to the present invention.

[0029]FIG. 7 is a timing diagram of related signals and clock of FIG. 6.

[0030]FIG. 8 is a schematic diagram of converting curves of each comparison unit in FIG. 6.

[0031]FIG. 9 is a schematic diagram of interpolated converting curves of comparison units in FIG. 6.

[0032]FIG. 10 is a schematic diagram of the data converter in FIG. 6 in another operating state.

[0033]FIG. 11 is a schematic diagram of interpolated converting curves of comparison units in FIG. 10.

DETAILED DESCRIPTION

[0034] Please refer to FIG. 6 of a schematic diagram of a data converter 30 in the present invention. The data converter 30 is an analog to digital data converter in a differential manner. In another words, the data converter 30 converts a difference of two analog input signals, which are Vin+ and Vin−, to a digital signal. The data converter 30 comprises a voltage dividing circuit 32, auxiliary circuits 36A and 36B, an encoding circuit 38, and a plurality of comparison units. For example, four comparison units 34A to 34D are shown in FIG. 6. To operate in a differential manner, the voltage dividing circuit 32 generates reference voltages Vr1 a to Vr4 a and Vr1 b to Vr4 b via resistors and biased circuits 32A and 32B. Since the voltage dividing circuit 32 is similar to those in the prior art, it is obvious without any detailed description.

[0035] Each comparison unit has the same structure. For example, the comparison unit 34B comprises eight switches S1 to S8 which are controlled by control signals AZ1, Az, and AZ2, wherein the switches in the comparison units 34A and 34C are controlled by control signals AZ1 a and AZ2 a and an inverted signal of Az. The switches S1, S2 and S5˜S8 can be transistors or transmission gates, and the switches S3 and S4 must be transmission gates. The comparison unit 34B also comprises a differential amplifier Qb comprising two differential input ends, a positive output end 42B and a negative output end 42A. The amplifier Qb comprises feedback circuits between both groups of I/O terminals that are controlled by switches S5 and S6 respectively. Both input ends are electrically connected to capacitors C. The other end of the capacitor C is controlled by switches and electrically connected to a reference voltage or an input signal alternatively. The comparison unit 34B further comprises a latch circuit 40 b for latching the amplifier Qb according to a clock vp and generating a corresponding digital signal. For matching the differential output of the amplifier Qb, the latch circuit 40 b also works in a differential manner. In another words, if the input at 44A is larger than the input at 44B, the latch circuit 40 b outputs a digital signal in a high state. In the same manner, a digital signal in a low state is output if the input at 44B is larger than the input at 44A. Please note that some electric routes are placed between the latch circuits of comparison units. For example, the input end 44A of the latch circuit 40 b is electrically connected to an input end of another latch circuit 40 a. The other input end 44B of the latch circuit 40 b is electrically connected to an input end of the latch circuit 40 c.

[0036] Please refer to FIG. 7; a timing diagram of the control signals Az, AZ1, AZ2, AZ1 a and AZ2 a and the clock vp is shown. The transverse axis in FIG. 7 is time. Each control signal controls corresponding switches to conduct in the high state and open in the low state. At time t3, the control signal Az controls corresponding switches to open so that closed loops are formed by the feedback circuits of the amplifier Qb of the comparison units 34B and the feedback circuits of the amplifier Qd of the comparison units 34D. Capacitors C of the comparison units described above are electrically connected to reference voltages Vr2 a, Vr2 b, Vr4 a, and Vr4 b respectively. The outputs of amplifiers Qb and Qd are electrically disconnected from the latch circuits 40 b and 40 d. At this time, the comparison units 34B and 34D are in the auto-zeroing process. A comparison unit in the auto-zeroing process charges the capacitor C according to the reference voltage so that the offset voltage is cancelled. Please notice the timing diagram shown in FIG. 7. When the control signal Az is in the low state, the comparison units 34B and 34D are in the auto-zeroing process. The switches of the feedback circuit that control these two amplifiers conduct at first, which means the control signal AZ2 raises to the high state, so that the capacitor is charged to cancel the offset voltage. After that, the control signal AZ1 raises to the high state and electrically connects the capacitors C to the reference voltages for charging the capacitors C.

[0037] In contrast with the comparison units 34B and 34D in the auto-zeroing process, the comparison units 34A and 34C are in the comparing process. At this time, the switches controlled by the control signal AZ2 a are opened and do not conduct to the feedback circuits of the amplifier Qa and Qc so that these two amplifiers are in open loops. The switches S3 and S4 controlled by the inverted signal of the control signal Az electrically connect the capacitor C to the input signals Vin+ and Vin−. The input signals are electrically coupled to the amplifier. Then, the differential input signals are compared with the reference voltage and the comparison result is output to the corresponding latch circuit in the differential manner. Therefore, the latch circuit generates corresponding digital signals to achieve the purpose of converting analog signals to digital signals.

[0038] As described above, the data converter 30 in the present invention is divided into two groups arranged in an interlaced manner. At the same time, half of the comparison units perform the auto-zeroing process and the other half performs the comparing process. To maintain the normal operating functions of the data converter 30, an active interpolation is performed to generate the output of the comparison units that perform the auto-zeroing process.

[0039] Please refer to FIG. 8 of a schematic diagram of converting curves of positive output ends and negative output ends of the amplifiers Qa to Qd. The transverse axis of FIG. 8 is differential input voltage. The longitudinal axis of FIG. 8 is output voltage. Converting curves T1 to T4 are the converting curves of the positive ends of the amplifiers Qa to Qd and the converting curves T1 i to T4 i are the converting curves of the negative ends of the amplifiers Qa to Qd. A reference voltage Vr1 marked on the transverse axis corresponds to reference voltages Vr1 a and Vr1 b. In the same manner, reference voltages Vr2, Vr3, and Vr4 also correspond to Vr2 a, Vr2 b, Vr3 a, Vr3 b, Vr4 a, and Vr4 b. The comparing process of the input signals and the reference voltage of the amplifiers are performed as following. When the differential input signals of the comparison unit, for instance 34A, are larger than the reference voltage Vr1 such as Vd1, the output voltage Voi of the positive end of the amplifier Qa, which is described in the converting curve T1 i, is larger than the output voltage Vo of the negative end, which is described in the converting curve T1. The latch circuit 40 a responds to the output voltage Voi and Vo to generate an output in a differential manner, for instance in a high state. If the differential input voltage Vd2 is less than the reference voltage Vr1, the output voltage of the negative end is larger than the output voltage Vo of the positive end. The latch circuit 40 a generates an output in a low state.

[0040] The intersection of the converting curves of output of the positive and corresponding negative ends are the reference voltages in the transverse axis used to trigger the corresponding latch circuit by the amplifier so that the state of the output signals is changed. Next, please refer to FIG. 9 of a schematic diagram of replacing the comparison unit 34B in an active interpolation. The transverse axis and longitudinal axis are the same as FIG. 8. Converting curves T1, T2, T2 i and T3 are respectively related to a positive output end of the amplifier Qa, a positive output end of the amplifier Qb, a negative output end of the amplifier Qb and a positive output end of the amplifier Qc. When the comparison unit 34B performs the auto-zeroing process, the negative output end of the amplifier Qa and the positive output end of the amplifier Qc are electrically connected to the differential input end of the latch circuit 40 b as shown in FIG. 6. At this time, output digital signals of the latch circuit 40 b are determined by output of the negative output end of the amplifier Qa and the positive output end of the amplifier Qc. Converting curves of these two output ends are illustrated as the converting curves T1 i and T3 in FIG. 9. If the reference voltage Vr2 is the interpolation of the reference voltage Vr1 and Vr3, which can be achieved by proper design, the intersection of the converting curve T1 i and T3 is corresponding to the reference voltage Vr2 on the transverse axis. The converting characteristic of the interpolation of the converting curves T1 i and T3 is different from the converting characteristics of the original converting curves T2 and T2 i of the amplifier Qb., However, the latch circuit corresponding to the amplifier Qb is driven in a differential manner. Thus, the converting curves T1 i and T3 intersect at the reference voltage Vr2 on the transverse axis correctly and the latch circuit operates correctly. If the differential signal is larger than the reference voltage Vr2, the output signal of the negative output end of the amplifier Qa is larger than the output voltage. If the comparison unit 34B performs the auto-zeroing process, the latch circuit 40 b, which receives the output of the negative output end of the amplifier Qa and the positive output end of the amplifier Qc, generates a digital signal in the high state. Relatively, if the differential input signal is less than the reference voltage Vr2, the output voltage of the negative output end of the amplifier Qa is less than that of the positive output end of the amplifier Qc, and the latch circuit 40 n generates a digital signal in the low state. The comparison unit Qb that is performing the auto-zeroing process, is replaced by an interpolation of the output of the amplifier Qa and Qc instead of the average of resistor network.

[0041] As shown in FIG. 6 and FIG. 7, when half of the comparison units perform the auto-zeroing process, the other comparison units perform the comparison process for comparing the differential input signal with a reference voltage of each comparison unit. The corresponding latch circuit generating a digital signal based upon the comparison result. At the same time, the comparison units performing the comparison process generate digital signals for the comparison units performing auto-zeroing process. As shown in FIG. 6, the comparison units 34A and 34C perform the comparing process and the other comparison units 34B and 34D perform the auto-zeroing process at time t3 shown in FIG. 7. The comparing function of the comparison unit 34B is replaced by using an active interpolation between the comparison units 34A and 34C. The comparing function of the comparison unit 34D is replaced by using an active interpolation between the comparison unit 34C and the auxiliary circuit 36B. In practice, the auxiliary circuit 36B can be also a comparison unit. In this design, the auxiliary circuit is similar to a circuit between the comparison unit 34A to 34D, but the latch circuit in the auxiliary circuit 36B can be an equivalent loading circuit because the auxiliary circuit does not need to generate digital signals but provides an output voltage for the amplifier to interpolate actively with the comparison unit 34C for replacing the comparison unit 34D in the auto-zeroing process.

[0042] Please refer to FIG. 10 of a schematic diagram of an operating state of the data converter 30 at the time t4 shown in FIG. 7. At this time, the comparison units 34A and 34 c perform the auto-zeroing process, and the comparison units 34B and 34D perform the comparing process. The comparing function of the comparison unit 34C is replaced by an active interpolation between the comparison units 34B and 34D and that of the comparison unit 34A is replaced by an active interpolation between the auxiliary circuit 36A and the comparison unit 34B. The auxiliary circuit 36A is substantially the same as the auxiliary circuit 36B. Please refer to FIG. 11 of a schematic diagram of replacing the comparing function of the comparison unit 34C by an active interpolation between the comparison units 34B and 34D. FIG. 11 is similar to FIG. 8 and FIG. 9. Converting curves T2, T3 and T4 i are converting curves of the negative output end of the amplifier Qb, the negative output end of the amplifier Qc, and the positive output end of the amplifier Qd. Please notice that the negative output end of the amplifier Qb in the comparison unit 34B and the positive output end of the amplifier Qd in the comparison unit 34D is used to replace the negative and positive output ends of the amplifier Qc of the comparison unit 34C. As known, the interpolation is different from the case shown in FIG. 9. If the converting curves T2 and T4 i intersect at the reference voltage Vr3 on the transverse axis, the converting curves T2 and T4 i are still able to drive the latch circuit 40 c in the comparison unit 34C correctly and a differential drive formed by the converting curves t3 and T3 i of the amplifier Qc is replaced.

[0043] In summary, the data converter 30 of the present invention comprises a group of comparison units performing the auto-zeroing process and another group of comparison units performing the comparing process to generate corresponding digital signals and outputs of the comparison units performing the auto-zeroing process with an active interpolating manner. In contrast to the prior art, the data converter in the present invention offers several advantages. It is more flexible. The present invention uses an active interpolating manner so that the circuit structure is simpler and the resistor network is not required. Thus, time and cost for the designing and manufacturing is relatively reduced, and the operation is more precise. The problem of matching incorrectly is therefore also solved. The active interpolating manner in the present invention is less sensitive to the non-linear converting curves of the amplifier. The active interpolation used in the present invention only requires the converting curves of the positive and negative output ends of neighboring interpolating amplifier to be odd symmetric to each other and intersect at correct positions for the latch circuit to be driven in a differential manner. Thus, the requirement of the linear range of the converting curves in the present invention is lower than that in prior art.

[0044] Moreover, the prior comparison units are switched between the auto-zeroing process and comparing process one by one. Therefore, each comparison unit needs a specific control signal. That means the number of the control signals increase when the number of comparison units increase. The conventional data converter requires a complicated circuit to generate enough control signals in different phase and circuit layout for transmitting those control signals to the corresponding comparison units also increases the circuit difficulty. When redesigning another circuit with a different number of comparison units, all the timing of the control signals and related circuits must be redesigned. In the present invention, one group of the comparison units performs the comparing process and the other group performs the auto-zeroing process. Only two groups of control signals are required to control switching of all comparison units between the comparing process and the auto-zeroing process in a interlaced sense. For instance control signals Az, AZ1 and AZ2 are a first group and the inverted signals are the other group in FIG. 7. No matter how many comparison units are used, a few group of control signals are required. The present invention simplifies circuit layouts and reduces the manufacturing costs.

[0045] Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

What is claimed is:
 1. A data converter for converting an input signal to a corresponding digital signal, the data converter comprising: a first, a second and a third comparison units for comparing the input signal with a first, a second and a third reference signals respectively to generate the corresponding digital signal; and a switch circuit connected to the output ends of the first comparison unit, the third comparison unit, and the second comparison unit; wherein when the second comparison unit performs an auto-zeroing process, a digital signal corresponding to the second comparison unit is generated from the outputs of the first comparison unit and the third comparison unit.
 2. The data converter of claim 1 wherein each of the first, the second and the third comparison units comprises a latch circuit for outputting the digital signal.
 3. The data converter of claim 1, wherein each of the comparison units comprises an amplifier for amplifying a voltage difference between the input signal and the corresponding reference signal so as to generate a corresponding positive output and a corresponding negative output.
 4. The data converter of claim 3, wherein each of the comparison units further comprises a feedback circuit electrically connected between an output end and an input end of the amplifier, and when the comparison unit performs the auto-zeroing process, the feedback circuit is conducted.
 5. The data converter of claim 1, wherein the data converter further comprises a voltage dividing circuit for generating the first, the second, and the third reference signals.
 6. A data converter for converting an input signal to a plurality of digital signals, the data converter comprising: a first and a second comparison unit arrays, each of the comparison unit arrays comprising a comparison unit and an auxiliary comparison unit, respectively comparing the input signal with a corresponding reference signal and generating each of the digital signals in a differential manner; wherein the digital signal of a comparison unit array in an auto-zeroing is generated by outputs of the associated comparison units in another comparison unit array in data converting via active interpolation.
 7. The data converter of claim 6, wherein each of the comparison units comprises an amplifier for amplifying a voltage difference between the input signal and the corresponding reference signal, the amplifier having a positive output end and a negative output end used as a positive output end and a negative output end of the comparison unit respectively.
 8. The data converter of claim 7, wherein each of the comparison units further comprises a latch circuit coupled with the positive output end and the negative output end so as to output the digital signal. 